SMF-DTW and MSP-GSO are former routes that would be at top of my list of both highly useful and not completely unlikely to be added/return.
For things that are unlikely to happen but potentially useful, I'd vote for SMF-CVG, SLC/LAX-GSO, and LAX-CLT.
Internationally, I would also love to see frequency/seasonality increased on some of the LAX-Central American routes, such as LAX-LIR. And selfishly, any service to NZ would be welcomed, maybe SEA-AKL would work on a 2-3 times a week frequency since LAX is already pretty saturated.